In communications cables which comprise a plurality of pairs of individually insulated metallic conductors, interference between circuits occurs because of the proximity of the paired circuits. One form of interference is referred to as crosstalk which is the induction of current in one conductor pair, which is commonly called the disturbed pair, by currents flowing in another pair, which is commonly called the disturbing pair.
Various arrangements have been used to reduce crosstalk. One such arrangement includes the use of varying twist lengths among the conductor pairs of a cable. Another arrangement includes a metallic shield that is applied over single conductor pairs of groups of pairs which are then assembled together to form a cable core. Also, cable cores have been assembled by stranding together particularly ones of the conductor pairs to form a center portion, enclosing those pairs that form the center portion with a shield which is often referred to as a screen, stranding additional conductor pairs around the center portion, then applying an outer shield and other sheathing components.
The advent of more recently developed transmission techniques imposed additional requirements on cable shields. In one kind of presently manufactured communications cable, a predetermined number of conductor pairs are formed into a unit and a cable core is formed to include a plurality of these units. With one of the new transmission techniques, the units of a cable core are divided into two groups, one carrying signals in one direction, and the other, signals in opposite direction.
In order to enable signals of the same of different frequencies to be transmitted in opposite directions within a single communications cable, especially when the signals are at carrier frequencies, it is necessary to shield the cable conductors of one group which are used for transmission in one direction electromagnetically from the conductors of a second group which are used for transmission in the opposite direction. Such shielding reduces crosstalk which may occur, for example, when the attenuated currents arriving at a repeater are unshielded from the amplified currents leaving the repeater and entering the other conductors of the cable.
The shielding of one group of cable units from the second group includes a screen that is disposed internally within the core to separate the two groups from each other and that forms part of the cable core. The employment of internal cable shields in communications cables to improve crosstalk characteristics between two groups of conductors therein is taught in U.S. Pat. No. 1,979,402 to H. Nyquist. Of more recent vintage, U.S. Pat. No. 3,803,340 discloses an internal cable shield including a metallic strip formed longitudinally in and with the cable core during a stranding operation in order to separate one group of conductors in the core from another group therein.
One improvement to cable screens has been the utilization of an internal cable shield comprising a plastic-coated metallic strip with the plastic overlying the edges of the metallic strip on the opposite sides thereof. See U.S. Pat. No. 3,622,683. The plastic coating on the metallic strip which is to form the internal shield acts as a dielectric to keep unwanted currents, which may be emitted from pinholes in the insulation of individual insulated conductors, from reaching the metallic strip portion of the shield.
Screens of various configurations have been used with one type being shown in above-mentioned U.S. Pat. No. 3,803,340. In a screen having an S or Z-shaped configuration, not only does the screen separate the groups of units but it has arcuately shaped end portions, each one of which extends around an associated group of units with its outwardly facing surface confronting the inwardly facing surface of an outer metallic shield of the cable.
During the manufacture of a recently designed cable an S-shaped screen comprising a plastic-aluminum laminate is interposed between two groups of cable units. The units and the laminate are advanced through a first facelate of a stranding apparatus where they are separated into two groups with the laminate being partially formed into the final screen configuration. As the units and the laminate are advanced through a second faceplate, the continued formation of the laminate toward its S-shape blocks the lower group of cable units from view by the operator and prevents a visual check of the position of the units within the group. This presents a problem since the position of each unit within its associated group is important to the structure of the cable.
Another problem which occurs during the formation of an internal cable shield is the tendency of the metallic strip from which the shield is formed to buckle as it is caused to enclose the groups of units. The extent of this problem depends on the configuration of the internal shield and the relative imbalance if any between the number and position of the units within each group.
What is needed and what the prior art does not show is the capability of maintaining cable units within a predetermined arrangement while forming a shield that is disposed between groups of the units.